Recovery of oil from subterranean formations frequently involves displacing crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) would provide a bank of oil of substantial depth being driven to a producing well; in practice, that frequently is not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable to a driving fluid than others. As a consequence, channeling frequently occurs so that the driving fluid flows preferentially through zones depleted of oil (so-called "thief" zones) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable. High permeability zones can also cause undesirable loss of drilling fluids when a well (e.g., water, oil or waste disposal) is being drilled. Misplaced casing perforations or casing leaks are another cause of channeling of the driving fluid through zones of high permeability in the subterranean formations. In addition, casing leaks sometimes occur in the annular region above the injection or production packer, and need to be dealt with whether the leaks occur in high or low permeability zones.
A variety of fluid diversion techniques have been proposed in the prior art. Typically, a gel is formed in situ in zones of very high permeability, thus plugging them and causing fluid to flow through zones which originally were of lower permeability than those which have been plugged. However, such techniques are in general not suitable at temperatures in excess of about 100.degree. C. and are limited to short gel times, e.g., less than 12 hours, thereby severely limiting their effectiveness. Thus, the prior art fluid diversion gels cannot be used in high temperature reservoirs commonly encountered in deep wells or in steam-flood operations, and in many cases the treatment composition can be pumped only a short distance from the wellbore before it gels regardless of reservoir temperature.
It has been proposed that sodium silicate gels be used to plug subterranean permeable zones. For example, U.S. Pat. No. 1,421,706 discloses that an aqueous solution of sodium silicate can be reacted with alkaline earth metal compounds, or with HCl, to produce cementing precipitates in wells. Similarly, in U.S. Pat. No. 3,202,214, aqueous sodium silicate solutions are reacted with an agent that will cause gelling of the sodium silicate solution, for example a substance that will liberate hydrogen ions, an agent which undergoes the Cannizzaro reaction, or a mixture of a reducing agent and an oxidizing agent. Hess, in U.S. Pat. No. 3,850,249, discloses a method for treatment of a permeable formation by delayed gelling of an acid-gelable liquid, gelation being delayed by latent pH adjustment to a pH of less than 5 at a temperature in the range of -10.degree. to 175.degree. C. The patent examples demonstrate gel times of less than one day at 80.degree. C. and the claims are limited to organic settable liquids having gel times of 1-24 hours. In U.S. Pat. No. 4,015,995 (a divisional of 3,850,249), Hess discloses and claims aqueous or alcoholic inorganic silicates as the acid-gelable material; however, none of the patent examples relates to any such silicate. The use of sodium silicate for this purpose is not without its problems. Competing with the availability of the silicate gel to plug the formation is the tendency of the silicate to coat particulate matter in the formation rather than forming a space-filling gel that plugs the interstices. Moreover, sodium silicate gels are not suitable at high temperatures and their gel times are too short for many operations. In addition, silicates are injected in wells at either a high or a low pH, not near neutral. That means that reactions which are usually undesirable, such as ion exchange and mineral dissolution, will proceed more rapidly than at a neutral pH. Also, the gel time of sodium silicate solutions is extremely sensitive to changes in pH, temperature, ionic strength, concentration, and the like. Furthermore, silicates at high ionic strengths are ineffective for forming gels. Small variations in salt concentration, pH, and polyvalent metal cations at low concentation, such as calcium and magnesium, can cause premature gelling, or delay gelling long after the desired gelling time. Because of the heterogenous nature of underground formations, such small variations will occur frequently and the excessive sensitivity of silicate gels to such variation makes their control in practical use very difficult.
Christopher, in U.S. Pat. No. 3,965,986, discloses a method for plugging zones of high permeability in which an aqueous slurry of colloidal fumed silica is combined in the high permeability zone with a surfactant or surfactant solution. The aqueous slurry of colloidal fumed silica may be pumped into the high permeability zone first followed by the aqueous surfactant or vice versa. Like the others in prior art, there are disadvantages to the Christopher technique. Christopher discloses that colloidal fumed silica is agglomerated to the extent that even after deagglomeration in a Cowles high speed mixer, it is necessary to continue mixing the slurry at a slow speed to prevent settling, and the continued mixing notwithstanding, it is also necessary to use the colloidal silica within 48 hours. Many formations which must be sealed consist of relatively small pores even though they may still be more permeable than desired. The aggregated compositions of Christopher may not be able to penetrate such formations. In contrast, the disaggregated sols of this invention can enter even the smallest or tightest formations which it might be desirable to plug. Such pores are of the order of a micron or larger (i.e., 10.sup.-4 centimeters) whereas the colloidal particles of this invention are only 4.times.10.sup.-3 to 0.1 microns (i.e., 4.times.10.sup.-7 to 10.sup.-5 centimeters) in diameter. The fact that Christopher's silica compositions must be stirred continuously to avoid settling indicates that they contain particles in excess of a micron in diameter and thus could not enter the smallest pore formations. Moreover, Christopher's technique requires mixing of the fumed silica with the surfactant in the subterranean formation. Effective mixing would be difficult if not impossible to obtain; only the fluids at the interface will mix. Apparently to minimize disadvantages of ineffective mixing, Christopher proposes alternately injecting small quantities of surfactant and fumed silica. However, if one of the first slugs of fumed silica should gel upon contact with the surfactant, further pumping of alternating slugs of fumed silica and surfactant would be impeded, or blocked entirely, thus making it impossible to effect blocking of permeable zones except those that are immediately adjacent to the well bore. Furthermore, fumed silica, being substantially non-hydrated, can be very difficult to disperse in water.